High power semiconductor package with conductive clip

ABSTRACT

One exemplary disclosed embodiment comprises a high power semiconductor package configured as a buck converter having a control transistor, a sync transistor, a driver integrated circuit (IC) for driving the control and sync transistors, and a conductive clip electrically coupling a sync drain of the sync transistor to a first leadframe pad of the package, wherein the first leadframe pad of the package is electrically coupled to a control source of the control transistor using a wirebond. The conductive clip provides an efficient connection between the control source and the sync drain by direct mechanical connection and large surface area conduction. A sync source is electrically and mechanically coupled to a second leadframe pad providing a high current carrying capability, and high reliability. The resulting package has significantly reduced electrical resistance, form factor, complexity, and cost when compared to conventional packaging methods using wirebonds for transistor interconnections.

BACKGROUND OF THE INVENTION

The present application claims the benefit of and priority to a pending provisional application entitled “Metal Clip for Efficient, Low Cost Package with Improved Current Carrying Capability and Reduced Form Factor and with Application in Buck Converters,” Ser. No. 61/460,553 filed on Jan. 3, 2011. The disclosure in that pending provisional application is hereby incorporated fully by reference into the present application. Additionally, pending applications Ser. No. 11/986,848, filed on Nov. 27, 2007, titled “Synchronous DC/DC Converter,” and Ser. No. 12/928,102, filed on Dec. 3, 2010, titled “DC/DC Converter with Depletion-Mode III-Nitride Switches,” are also incorporated fully by reference into the present application.

DEFINITION

In the present application, “III-nitride” (or “III-Nitride”) refers to a compound semiconductor that includes nitrogen and at least one group III element, such as, but not limited to, GaN, AlGaN, InN, AlN, InGaN, InAlGaN and the like.

1. Field of the Invention

The present invention relates generally to semiconductor devices. More particularly, the present invention relates to packaging of semiconductor devices.

2. Background Art

For optimization of form factor, performance, and manufacturing cost, it is often desirable to integrate the components of a power circuit, such as a half-bridge based DC-DC converter or buck converter, into a single compact package. Thus, several package designs, including quad flat no leads (QFN) packages, have been developed to integrate several transistors within a single compact package. To provide sufficient electrical performance for the reliable operation of high power semiconductor packages, it is crucial to ensure high current carrying capacity and low resistance between transistors within the package.

Unfortunately, conventional high power semiconductor package designs use wirebonds for transistor interconnections, undesirably increasing electrical resistance while reducing current carrying capacity. Additionally, by following conventional package design rules to successfully accommodate such wirebonds, package form factor and complexity may undesirably increase. For example, package height may be increased to provide sufficient wirebond loop height, lateral package size may be increased to avoid wire crossing and the potential for wire shorting, and additional area on the package may be reserved for bond pad connections, thereby undesirably reducing package power density. Additionally, the increased package complexity resulting from the wirebonds may negatively affect manufacturing time, cost, and package yields.

Thus, a unique and cost-effective solution is needed to support the efficient design and operation of high power semiconductor packages integrating multiple transistors, such as buck converters.

SUMMARY OF THE INVENTION

A high power semiconductor package with conductive clip, substantially as shown in and/or described in connection with at least one of the figures, and as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a circuit diagram of a buck converter using a half-bridge topology.

FIG. 1B illustrates a top view of a conventional high power semiconductor package.

FIG. 2A illustrates a top view of a high power semiconductor package with a conductive clip according to an embodiment of the invention.

FIG. 2B illustrates a cross sectional view of a portion of a high power semiconductor package according to an embodiment of the invention.

FIG. 2C illustrates a cross sectional view of a portion of a high power semiconductor package according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present application is directed to a high power semiconductor package with conductive clip. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order not to obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detailed description are directed to merely exemplary embodiments of the invention. To maintain brevity, other embodiments of the invention, which use the principles of the present invention, are not specifically described in the present application and are not specifically illustrated by the present drawings.

FIG. 1A illustrates a circuit diagram of a buck converter using a half-bridge topology. Diagram 100 includes switched node 115, driver integrated circuit (IC) 120, control transistor 140 (also referred to as a “control switch” or a “control FET”), and synchronous transistor 160 (also referred to as a “synchronous switch,” a “sync switch,” a “synchronous FET,” or a “sync FET”). The source of control transistor 140 is coupled to the drain of sync transistor 160 at switched node 115. Driver IC 120 operates on voltage Vdr and controls the duty cycles of control transistor 140 and sync transistor 160, thereby converting the input voltage Vin to a specific output voltage Vout. Control transistor 140 and sync transistor 160 may each comprise a conventional field effect transistor (FET) switch, for example a silicon FET. However, control transistor 140 and sync transistor 160 may each also comprise a non-silicon FET or any other FET in general. Alternatively, one or both of control transistor 140 and sync transistor 160 may also comprise a III-nitride transistor.

As discussed above, it may be desirable to implement the circuit of diagram 100 of FIG. 1A in a compact and integrated package, such as a QFN package. Accordingly, turning to FIG. 1B, FIG. 1B illustrates a top view of a conventional high power semiconductor package. Package 110 of FIG. 1B includes leadframe 112, leadframe paddle 113, wirebonds 114 a, 114 b, 114 c, and 114 d, driver IC 120, control transistor 140, and sync transistor 160. Package 110 may comprise, for example, a QFN package. Control transistor 140 includes a top surface having a control gate 142 and a control source 144. Control transistor 140 also includes a bottom surface having a control drain 146, hidden from view in FIG. 1B. Sync transistor 160 includes a top surface having a sync gate 162 and a sync source comprised of sync source pads 164 a, 164 b, 164 c, 164d, and 164 e. Sync transistor 160 also includes a bottom surface having a sync drain 166, hidden from view in FIG. 1B.

The sync source of sync transistor 160 is electrically coupled to leadframe 112 by several wirebonds connected to various sync source pads such as, for example, wirebonds 114 d connected to sync source pad 164 c. Driver IC 120 is connected to several sections of leadframe 112 for input/output. Driver IC 120 is also electrically coupled to control gate 142 via wirebond 114 b and sync gate 162 via wirebond 114 a. Sync drain 166 is electrically coupled to leadframe paddle 113 of leadframe 112, and leadframe paddle 113 in turn is electrically coupled to control source 144 through wirebonds 114 c. Similarly, control drain 146 may also be disposed on leadframe paddle 113, hidden from view in FIG. 1B. Leadframe paddle 113 may comprise an easily solderable metal such as aluminum, or other solderable materials such as a metal alloy or a tri-metal.

Thus, the layout of package 110 in FIG. 1B connects driver IC 120, control transistor 140, and sync transistor 160 as shown in diagram 100 of FIG. 1A. As previously noted, for high power semiconductor packages, it is particularly important to optimize the interconnections between transistors, such as at switched node 115 of FIG. 1A. However, the conventional package design shown in FIG. 1B requires the use of wirebonds 114 c to connect control transistor 140 and sync transistor 160 at switched node 115 of FIG. 1A. Moreover, the current input/output path for sync source pads 164 a through 164 e must travel through restrictive wirebonds as well, such as wirebonds 114 d. The wirebonds of package 110 in FIG. 1B thus disadvantageously increase package electrical resistance, form factor, complexity, and cost.

Moving to FIG. 2A, FIG. 2A illustrates a top view of a high power semiconductor package with a conductive clip according to an embodiment of the invention. Package 210 in FIG. 2A may comprise, for example, a leadless package such as a QFN package. Package 210 includes leadframe pads 212 a, 212 b, 212 c, 212 d, and 212 e, wirebonds 214 a, 214 b, and 214 c, IC driver 220, control transistor 240, sync transistor 260, and conductive clip 280. Control transistor 240 includes a top surface having a control gate 242 and a control source 244. Control transistor 240 also includes a bottom surface having a control drain 246, hidden from view in FIG. 2A. Sync transistor 260 includes a top surface having a sync drain 266. Sync transistor 260 also includes a bottom surface having a sync gate 262 and a sync source comprised of sync source pads 264 a, 264 b, 264 c, 264 d, and 264 e. Sync gate 262 and sync source pads 264 a through 264 e are further arranged into a grid. However, alternative embodiments may use other pad arrangements, such as an L-shaped sync source pad. Significantly, it is noted that sync transistor 260 is reversed in orientation (sync transistor 260 has drain on top, with source and gate on bottom) in relation to sync transistor 160 of FIG. 1B (sync transistor 160 has source and gate on top, with drain on bottom). It is noted that in various embodiments of the present invention, one or both of control transistor 240 and sync transistor 260 can be depletion mode transistors, for example, III-nitride depletion mode transistors.

For purposes of clarity, an encapsulating mold compound or a hermetic seal is omitted from FIGS. 2A, 2B, and 2C, but may be included in package 210. Furthermore, with respect to FIG. 2A, driver IC 220 may correspond to driver IC 120 from FIG. 1A, control transistor 240 may correspond to control transistor 140 from FIG. 1A, and sync transistor 260 may correspond to sync transistor 160 from FIG. 1A.

As shown in FIGS. 2A and 2B, conductive clip 280 electrically couples sync drain 266 and leadframe pad 212 a. In turn, leadframe pad 212 a is electrically coupled to control source 244 through wirebonds 214 c. Thus, sync drain 266 is connected to control source 244 using the direct mechanical connection and large surface area conduction of conductive clip 280 in FIG. 2A, in conjunction with wirebonds 214 c. Conductive clip 280 may comprise a metal such as copper, a metal alloy, or another highly conductive material, and may be attached to sync drain 266 and leadframe pad 212 a using solder, conductive adhesive, or another attachment means.

Thus, package 210 of FIG. 2A may also implement the buck converter of FIG. 1A, but with far greater package performance compared to package 110 of FIG. 1B. More specifically, conductive clip 280 provides a low resistance, high current path for the connection at switched node 115 of FIG. 1A, while the sync source, i.e. source of sync transistor 260, is connected to ground through the low resistance, low inductance, and reliable mechanical connection provided by leadframe pad 212 b, instead of the restrictive wirebonds 114 d of FIG. 1B, thereby advantageously reducing package electrical resistance, form factor, complexity, and cost.

FIG. 2B illustrates a cross sectional view of a portion of a high power semiconductor package according to an embodiment of the invention. The portion shown in FIG. 2B corresponds to the cross sectional line indicated by line 2B-2B of FIG. 2A. FIG. 2B includes leadframe pads 212 a, 212 b, and 212 e, sync gate 262, sync source pads 264 b and 264 d, sync transistor 260, sync drain 266, and conductive clip 280. While only the semiconductor device body is indicated as sync transistor 260 for simplicity, it is to be understood that sync transistor 260 may also include any top and bottom surface electrodes such as sync drain 266, sync gate 262, and sync source pads 264 b and 264 d.

Comparing FIG. 2B with FIG. 2A, it can be seen that the sync gate 262 is electrically coupled to leadframe pad 212 e, where leadframe pad 212 e is in turn electrically coupled to IC driver 220 through wirebond 214 a. Additionally, FIG. 2B makes more apparent the L-shape of conductive clip 280, which enables the electrical coupling of sync drain 266 to leadframe pad 212 a. As previously discussed, leadframe pad 212 a may then be electrically coupled to control source 244 through wirebonds 214 c. The direct electrical and mechanical coupling of the source of sync transistor 260 to leadframe pad 212 b is also illustrated in FIG. 2B. Thus, more efficient current conduction is provided to and from external connections of the package, such as leadframe pad 212 b.

With respect to FIG. 2C, FIG. 2C illustrates a cross sectional view of a portion of a high power semiconductor package according to an embodiment of the invention. The portion shown in FIG. 2C corresponds to the cross sectional line indicated by line 2C-2C of FIG. 2A. FIG. 2C includes leadframe pads 212 b, 212 d, and 212 e, IC driver 220, sync gate 262, sync source pad 264 a, sync transistor 260, sync drain 266, and conductive clip 280. Comparing FIG. 2C with FIG. 2A, FIG. 2C makes more apparent the orientation of sync transistor 260 and the routing of leadframe pads 212 b, 212 d, and 212 e.

According to the present invention, by using a direct leadframe pad connection for the sync source, and by using conductive clip 280 for the connection between the control source 244 and the sync drain 266, a package with reduced electrical resistance, form factor, complexity, and cost may be achieved when compared to conventional packaging methods using wirebonds such as wirebonds 114 d of FIG. 1B. Additionally, the large surface area provided by leadframe pad 212 b, and conductive clip 280 in conjunction with leadframe pad 212 a allows for more efficient input and output current conduction.

From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without depting from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skills in the art would recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. As such, the described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein, but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention. 

The invention claimed is:
 1. A high power semiconductor package comprising: a control transistor including a control source and a control gate on a top surface thereof, and a control drain on a bottom surface thereof; a sync transistor including a sync drain on a top surface thereof and a sync gate and a sync source on a bottom surface thereof; a driver integrated circuit (IC) electrically coupled to said control gate and said sync gate; a conductive clip electrically coupling said sync drain to a top surface of a first leadframe pad of said high power semiconductor package; said sync source being coupled to a second leadframe pad; said control source being coupled to said top surface of said first leadframe pad using a wirebond; wherein said control transistor and said driver IC are situated along a common side of said sync transistor.
 2. The high power semiconductor package of claim 1, wherein said first leadframe pad is electrically coupled to said control source using said wirebond.
 3. The high power semiconductor package of claim 1, wherein said sync source is mechanically and electrically coupled to said second leadframe pad.
 4. The high power semiconductor package of claim 1, wherein said package comprises a buck converter.
 5. The high power semiconductor package of claim 1 , wherein said package is a leadless package.
 6. The high power semiconductor package of claim 1, wherein said sync transistor includes a sync source arranged into a grid.
 7. The high power semiconductor package of claim 1, wherein said conductive clip is attached to said sync drain by solder.
 8. The high power semiconductor package of claim 1, wherein said driver IC is electrically coupled to said control gate using a wirebond.
 9. The high power semiconductor package of claim 1, wherein said sync gate is attached to a third leadframe pad of said package, said third leadframe pad electrically coupled to said driver IC using a wirebond.
 10. The high power semiconductor package of claim 1, wherein said conductive clip comprises a copper clip.
 11. The high power semiconductor package of claim 1, wherein said control transistor comprises a III-nitride depletion mode transistor.
 12. The high power semiconductor package of claim 1, wherein said sync transistor comprises a III-nitride depletion mode transistor.
 13. A high power semiconductor package comprising: a control transistor including a control source on a top surface thereof; a sync transistor including a sync drain on a top surface thereof; a driver integrated circuit (IC) driving said control transistor and said sync transistor; a conductive clip electrically coupling said sync drain to a top surface of a first leadframe pad of said high power semiconductor package, said control source being electrically coupled to said top surface of said first leadframe pad using a wirebond; wherein said control transistor and said driver IC are situated along a common side of said sync transistor.
 14. The high power semiconductor package of claim 13, wherein said sync transistor includes a sync source that is mechanically and electrically coupled to a second leadframe pad.
 15. The high power semiconductor package of claim 13, wherein said package comprises a buck converter.
 16. The high power semiconductor package of claim 13, wherein said package is a leadless package.
 17. The high power semiconductor package of claim 13, wherein said sync transistor includes a sync source and a sync gate arranged into a grid on a bottom surface thereof.
 18. The high power semiconductor package of claim 13, wherein said driver IC is electrically coupled to a control gate on said top surface of said control transistor using a wirebond.
 19. The high power semiconductor package of claim 13, wherein said sync transistor includes a sync gate on a bottom surface thereof, said sync gate attached to a third leadframe pad of said package, said third leadframe pad electrically coupled to said driver IC using a wirebond.
 20. The high power semiconductor package of claim 13, wherein each of said control transistor and said sync transistor comprises a III-nitride depletion mode transistor. 